Plant development is a complex physiological and biochemical process requiring the co-ordinated expression of many genes. The production of new plant varieties with improved agricultural or commercial qualities can be achieved by modifying this coordinated pattern of gene expression. Such modifications have been achieved by conventional plant breeding techniques. However, the exact changes in gene expression that result in the production of the improved variety have not been readily characterised. More recently, recombinant DNA techniques have been used to modify the expression patterns of individual, specific plant genes without directly affecting the expression of other plant genes. In this way, the expression pattern of an individual gene can be either enhanced or inhibited either in the whole plant or in specific tissues or developmental stages.
The inhibition of specific individual plant genes has been achieved by the introduction into the plant of novel genes designed to express RNA homologous, in part, to the endogenous plant gene. In several cases, it has been demonstrated that expression of the target gene can be inhibited by two different strategies. These involve the introduction of specific genes designed to express either antisense or sense RNA. A typical example is the down-regulation of the gene encoding the tomato fruit cell wall enzyme, polygalacturonase, by the expression of either antisense RNA (Smith et al 1988 Nature 344, 724-726) or sense RNA (Smith et al 1990 Mol Gen Genet 224, 477-481). A further example is the down-regulation of the gene encoding chalcone synthase in petunia by either sense or antisense RNA.
The mechanisms by which the expression of a specific gene is inhibited by either antisense or sense RNA genes are not clearly understood. It has been proposed that RNA-RNA duplex molecules may be formed within the cells resulting in the inhibition of expression. However, other and perhaps different mechanisms may operate for the two strategies for down-regulation. Specific individual genes have been inhibited by greater than 99% by the two strategies independently.